Classic & Vintage - Thread size 1977 Mercian seat tube bolt??

Anyone out there know what the threads size would be for a 1977 Mercian seat tube bolt-screw?It is the bolt that holds the seat post in place.I putting my bike back on the road,and I borrowed(dumb shi>!)the screw sometime in the distant past for some POS part or motorcycle application.It is 7mm(I think),but I don't know what the threads are.Thanks.CharliePS It seems it is more like 7mm,not 6mm like I said earlier.Could it be non metric??Maybe 9/32"??.

it may very well be sae thread not metric. i have a bob jackson and every time i adjust the seat i have to remember to use a 7/32 allen wrench instead of metric. i don't no the thread size but in trying to eyeball it i'd say 9/32s thread size. what may be of concern also is the bolt head diameter. my seat bolt head (round shape) screws into a recess so that it is flush with the hole. i think i have seen bolts like that at home depot in the spjecialty fastner area. nice bike! good luck. you might also want to check with the manufacturer if they have a website. they may still have the exact bolt.

Thanks.I found a bolt that has the right number of tpi or mm,but it is too loose-the diameter is just a bit too little.I think I will give Mercian a chance to answer,and then start fiddling again.I might have screwed up the threads a bit waaay back when-guess I'll find out.
Yes-mine also has the recessed head.I had to "turn"the head of this allen bolt down a bit on my $15000 lathe(harbor freight 4.5" $19.95 grinder) to make it slip in.Thanks.Charlie

Is the braze on really threaded? On many of the Mercian's they use a Campy seat post binder bolt, which does not thread into the braze on.

Yes,this SOB is threaded-I wish it wasn't.It would be a lot easier to find a Campy binder bolt-than finding this one.I might end up "converting" to to non threaded-I suspect that the threads aren't quite right.I might end up trying to use a Campy bolt "backwards".I have an allen bolt in there now that measures ~.275"-about 7mm.The threads per inch seem right,but it is too loose.Besides,I never heard of a 7mm bolt being used on a bike.??I have some 8mmbolts that go in about 1 1/2 turns-I wonder if maybe it is 8 mm,but tighter threading than the 8 x1.25 bolts??Hope Mercian comes thru.Thanks all.Charlie

Sounds like it could be either a 5/16"x 24 SAE bolt,(that's .31" diameter and 24TPI a fine thread) if it's a standard bolt (very possible for a British bike), but if it's metric and 8mm is the right diameter then it could be a 1mm thread pitch that's the "fine" thread versus the more typical 8X1.25 pitch. Both those options are available from a big fastener house like McMaster-Carr, and in different materials, too. I always go for stainless steel whenever I have the choice...I just hate rust. You could also take a tap of the correct size and run it through the fitting to clean up the threads.That's a nice bike!

unworthy1,I think you might be right on the 8x1.I'm pretty sure all my spare bolts are 8 1.25,since they are off Jap. motorcycles and that is a very common bolt for them.The 8 x1.25 go in about 11/2 turns and then stop-about what you would expect for a 1.25 going in a 1.
The threads almost certainly will need a little "help"-I'll have to see if my cheapo Chinese tap-die set has a 8x1mm.
Yes,it is a pretty bike.I reread a 70's vintage Mercian brochure on their web site.They specifically mention that the Vincitore "is extremely popular with our American customers."They sure had me pegged!Normally,I'm strictly function;no interest in looks,but those lugs just killed me.Thanks.Charlie